1. Field of the Invention
The present invention relates to a power transfer device for four-wheel drive vehicles, and more particularly to a power transfer device of the type which includes a limited-slip differential arranged to effect differentiation between the front and rear axles.
2. Description of the Prior Art
In Japanese Patent Early Publication No. 61-207219 issued on Sept. 13, 1986, there has been proposed a power transfer device adapted for use in combination with a power transmission, which transfer device comprises a first shaft rotatably mounted within a housing and having an input end for drive connection to an output shaft of the transmission and an output end for drive connection to a set of rear road wheels, a second shaft rotatably mounted within the housing in parallel with the first shaft for drive connection to a set of front road wheels, a drive member rotatably mounted on the first shaft and connectable thereto, a driven member rotatably mounted on the second shaft and connectable thereto, the driven member being drivingly connected to the drive member, and a pair of clutch sleeves each mounted on the first and second shafts and shiftable between a first position where both the clutch sleeves are retained to connect the drive and driven members to the first and second shafts for providing a four-wheel drive and a second position where both the clutch sleeves are retained to disconnect the drive and driven members from the first and second shafts for providing a two-wheel drive.
In operation of the power transfer device, differentiation between the front and rear axles is required when the front road wheels of the vehicle travel at a different distance that the rear road wheels, such as may occur in cornering. The differentiation, however, may not be effected in the power transfer device because of no provision of any differential, resulting in the occurrence of a tight corner braking phenomenon during travel of the vehicle if the four-wheel drive mode. Such a tight corner braking phenomenon will be avoided if the two-wheel drive mode is selected at the power transfer device prior to steering operation of the vehicle. It is, however, inconvenient to frequently select the two-wheel drive mode at the power transfer device at each time when the vehicle is steered. Although such problems can be solved by use of a differential of the bevel gear type at the power transfer device to effect the differentiation between the front and rear axles, the power transfer device becomes heavy and complicated in construction, and the differential will cause unpleasant gear noises in operation. In assembly of the differential, the first shaft of the transfer device must be divided into two sections for connection to both side gears of the differential. This results in an increase of the number of bearings for support of the first shaft and in a decrease of support rigidity of the first shaft.